The present invention relates to a valve unit for controlling braking pressure in at least one brake cylinder in a pneumatic braking system for wheeled vehicles, the valve unit being mounted in a conduit between a compressed air container and the brake cylinder, and comprising an inlet valve and an outlet valve which, under the action of at least one solenoid valve or the like are regulated by information signals from an electronic braking control system to regulate the air flow to and from the brake cylinder, said valve unit, during a pressure increase sequence being fed with pulses of constant length, thereby causing a stepwise pressure increase in the brake cylinder by opening and closing the inlet valve, said inlet valve comprising a regulating chamber which is delimited by a valve means and which is selectively in communication with either the valve unit inlet or with valve unit outlet to the ambient atmosphere.
The valve unit is regulated by an electronic unit incorporated in the braking control system. During a braking operation the electronic unit gives signals to the valve unit for raising, lowering or maintaining constant the braking pressure in the brake cylinder in question. The signals from the electronic unit are dependent on certain critical slip, retardation or acceleration values. It is important for the function of the system that the signals to the valve unit cause well-defined pressure alterations, particularly in regard to pressure increases, independent of whether the alteration is in conjunction with a high or low pressure in the brake cylinder. In other words, a major objective of the invention is the ability to cause a constant pressure increase in the brake cylinder for every pressure raising signal applied to the valve unit.
In regulating systems for pneumatic braking systems, a rise in pressure takes place quicker at lower pressure than at a higher pressure. Opening the inlet valve during a constant short period of time for increasing the brake cylinder pressure therefore gives a considerably larger pressure increase at low pressure in the brake cylinder than at a higher pressure. It is known to attempt the solution of this problem by introducing into the valve a pressure sensitive constriction which causes slower pressure rise at lower pressures in the braking cylinder, while pressure rises at higher pressures are not affected thereby. However, this causes the pressure raising steps to diminish successively during the later phase of the filling cycle. The introduction of such a pressure sensitive constriction in a valve unit also means added complication and an increase in cost of the braking control system in its entirety.